1. Field of the Invention
The present invention relates to a reflection sound compression apparatus for installation in a sound field controller which allows an arbitrary sound field such as those in halls, etc. to be generated in a conventional room.
2. Prior Art
With the current development of hall simulation technology using the computer and the use of trend toward a digital technology for acoustic devices, the need for sound field control has been rapidly increasing. For this sound field control, a device for generating a sound field is used by performing convolution of a musical signal and an impulse response (reflection series) of a hall, etc., called a sound field controller. Although the convolution performed in this sound field controller can be realized by a DSP (digital signal processor) or a discrete IC, there is a limitation in the length of impulse response (the number of reflections) which is performed convolution from performance of the existing DSPs and ICs, and thus the convolution is normally being used by adjusting (compressing) the impulse responses measured in practice at the renown halls, etc. and also determined with simulation calculations, etc.
An explanation will follow of an example of the conventional reflection compression apparatus which compresses the above-mentioned impulse response, with reference to drawings.
FIG. 3 shows a block diagram of a conventional reflection compression apparatus. In FIG. 3, numeral 10 represents a RAM memory circuit RAM (Random Access Memory) which stores an impulse response of hall, etc. determined by measurement or calculation; 11 represents a calculating circuit which calculates an average energy of the reflection sounds in the time interval from the impulse response stored in the memory circuit 10, and allocates the value at a position of the reflection sound at which the maximum value is obtainable within the time interval; 12 represents a setting circuit for setting the reflection sound determined by the circuit 11 on a sound field controller; 13 represents a sound field controller for producing a sound field by performing convolution of a musical signal and the reflection sound set by the setting circuit 12; 14 represents a group of speakers responsive to the output signal of the sound field controller 13; and S.sub.M represents musical signals reproduced by compact disks, etc.
FIGS. 4(A)-(C) are shows diagrams for exhibiting a method of calculation in the calculating circuit 11, in which FIG. 4(A) is a schematic diagram of impulse responses obtained by measurement or calculation followed by digital sampling, FIG. 4(B) illustrates a reflection sound determined by the calculation circuit 11 exhibiting the magnitude of reflection sound at Ei (i equals to 1-8), and FIG. 4(C) illustrates a reflection sound compressed into the practically processable number (in this case 6 pieces) at the sound field controller. Also, T as shown in FIG. 4 (B) represents a time interval in which the reflection sounds are extracted.
In the reflection sound compression apparatus as shown in FIG. 3, impulse responses as determined by the calculation for the simulation of impulse responses or sound ray method, etc. which were measured in real halls, etc. and are stored in the memory circuit. Then, the calculation circuit 11 calculates an average energy of reflection sound in a certain time interval as shown in FIG. 4, allocates the value at the position of the reflection sound at which it takes the maximum value within the time interval, and makes other reflection sounds zero. The method of calculation may be presented by a formula as follows: ##EQU1## where E.sub.i is a magnitude of reflection sound extracted in the time interval i as shown in FIG. 4, h (n) is an impulse response stored in the memory circuit 10, and n is a parameter representing time.
The number i as shown in the formula above is the number of reflection sounds which enable the convolution to be performed in the sound field controller 13.
The calculation above corresponds to (A) and (B) in FIG. 4, and is in reality compressed to the number of reflection sounds which make processing possible with the sound field controller. The method of this compression adopts, for instance, a way in which reflection sounds in a number possible to perform the convolution are taken in the order from a bigger sound from the reflection sounds compressed to (B) in FIG. 4.
In this way, the reflection sounds determined by the calculation circuit 11 are set in the sound field controller 13 by the setting circuit 12, thereby allowing a greater number of reflection sounds determined by measurement and calculation to be compressed to the number of reflection sounds which are actually processable.
However, with such a conventional reflection sound compression apparatus, there is no means to appraise the physical approximation level between the original impulse response and the reflection sound as determined, and that there is a problem such as setting data in the sound field controller by extracting the data without objectivity to a high degree so that this approximation level finally needs correction in accordance with a human psychological scale.